1. Field of the Invention
The present invention relates to a load moment indicator of a crane provided with a suspension means.
2. Description of the Related Art
A conventional art will be described taking a crane with an auxiliary sheave shown in FIG. 6 as an example.
In the figure, reference numeral 1 designates a self-traveling type (in the figure, a crawler traveling type is shown) crane body 1. A boom 2 is mounted on the crane body 1 to so as to be hoisted and lowered. An auxiliary sheave bracket 4 with an auxiliary sheave 3 is mounted, as an auxiliary suspending arm, at the extreme end of the boom 2.
On the crane body 1 are mounted a boom raising and lowering winch 5, a main winch 6 and an auxiliary winch 7. The boom 2 is driven to be hoisted and lowered by the boom raising and lowering winch 5 through a boom reeving rope 8 and a boom guyline 9.
A main hoist rope 10 drawn out of the main winch 6 is suspended from the extreme end of the boom to suspend a main hook 11 suspended by many ropes. By the main hoist means constituted as described above, the main winding and hoisting work for raising and lowering mainly a very heavy cargo at a low speed takes place.
On the other hand, an auxiliary hoist rope 12 drawn out of the auxiliary winch 7 is suspended from the auxiliary sheave bracket 4 to suspend an auxiliary hook 13 permanently. By the auxiliary hoist means constituted as described above, the auxiliary winding and suspending work for raising and lowering mainly a light cargo at a high speed takes place.
The main winding and suspending work and the auxiliary winding and suspending work are sometimes carried out simultaneously.
The overload preventive method of a crane provided with two kinds of suspension means of the main side and the auxiliary side as described above is disclosed, for example, in Japanese Patent Application Laid-Open No. Hei 11-246178 Publication. The tension of both the main and auxiliary hoist ropes 10 and 12 and the tension of the boom guyline 9 are respectively detected by a detector to calculate a main hoist load, an auxiliary hoist load, and the whole hoist load. Subsequently, when the hoist loads, and at least one of load factors obtained from the rated loads preset reach a fixed value, an automatic stop valve is operated to automatically stop the crane operation.
The rated load termed herein is the maximum hoist load obtained on the basis of the stability of a crane and the strength of constitutional members (normally, the rupture strength of a rope), which load is calculated for every work radius in advance and stored in a memory.
Even where the auxiliary sheave bracket 4 is replaced with the auxiliary sheave 3 as the auxiliary suspending arm, a jib which is longer than the former is mounted to be raised and lowered or at a fixed angle, or where both the auxiliary sheave bracket 4 and the jib are mounted, and the suspending work is carried out by three suspension means of the main side and the two auxiliary sides, the overload preventive method is basically the same as that described above.
Where the suspending work is carried out simultaneously by both the main side and the auxiliary side, the load value capable of being suspended by each side of ought to be varied in view of the load suspended by the other side. Despite this, the respective rated load is set as a fixed value without taking the other side into consideration, and therefore an operator cannot grasp the actual maximum weight to be suspended, resulting in an obstacle to effective work.
It is an object of the present invention to provide a load moment indicator of a crane making the most of the suspending ability of a main side and an auxiliary side at the maximum and capable of permitting an operator to clearly grasp the tolerance of the hoist load.
The load moment indicator of a crane according to the present invention comprises:
1) a first hoist means for carrying out a first suspending work, the first hoist means having a first winch, a first rope drawn out of the first winch and suspended from the extreme end of a boom, and a first hook suspended by the first rope;
2) a second hoist means for carrying out a second suspending work, the second hoist means having a second winch, a second rope drawn out of the second winch and suspended from a suspending arm, and a second hook suspended by the second rope; and
4) a calculator as a calculating means for carrying out processing of preventing an overload on the basis of first and second hoist loads carried by the first and second hoist means, and for calculating rated loads determined separately by the first and second hoist means, respectively, the rated load being obtained by reducing one hoist load out of said first and second hoist by a value related to the other of the first and second hoist loads.
In the case of the aforementioned crane shown in FIG. 6, the first hoist means corresponds to a main hoist means. With respect to other constitutions, the corresponding relationship will be described below. The first winch, the first rope, and the first hook correspond to a main winch, a main hoist rope, and a main hook, respectively. Similarly, the second hoist means, the second winch, the suspending arm, the second rope correspond to an auxiliary hoist means, an auxiliary winch, an auxiliary suspending arm, an auxiliary hoist rope, and an auxiliary hook, respectively.
Further, it is preferred that the rated loads be constituted by the following:
(a) a reference value of the first hoist means is set on the basis of a given reference value determined from a view of safety including the stability of a crane and the rupture strength of the rope, and
(b) a hoist load of the second hoist means is converted into a load component of the first hoist means to thereby calculate a conversion value, the conversion value being subtracted from the reference value of the first hoist means.
While a case is given in which the hoist load of the second hoist means is converted into the load component of the first hoist means, the reverse thereto will suffice also. In short, the way of thinking for obtaining the rated load is as follows:
(a) a reference value of one side is set, in advance, on the basis of a given base determined from a view of safety of the stability of a crane, the rupture strength of a rope, etc., and
(b) a hoist load of the hoist means in other sides is converted into a load component of the one side on the basis of a base of the one side, and the converted value is subtracted from a reference value of the one side.
According to the above-described device, the rated load of the one side can be varied according to the hoist load of other sides. Therefore, the maximum hoist load that can be suspended by both systems, irrespective of the single hoisting work time and the simultaneous hoisting work time of both systems, can be determined as the rated load.
Accordingly, it is possible to make the most of the suspending ability of both the systems and for an operator to clearly grasp how much can be suspended.
Where the reference values of both the main side and auxiliary side are set on the basis of the same base (for example, the crane stability), in both the systems, the suspending weight of other sides may be taken (subtracted) as the load component of the one side on the basis of a base of the one side to thereby obtain the rated load.